Attempts to express real space, in which imaging has been performed based upon real image data captured by a photographic device mounted on a mobile body, as virtual space through use of a computer have been disclosed in, for example, “Endo, Katayama, Tamura, Hirose, Watanabe, Tanigawa: ‘Building Image-Based Cybercities by Using Vehicle-Mounted Cameras’ (The Society Conference of the Institute of Electronics, Information and Communication Engineers (IEICE), PA-3-4, pp. 276 277, 1997), and “Endo, Katayama, Tamura, Hirose, Watanabe, Tanigawa: ‘Building Image-Based Cybercities by Using Vehicle-Mounted Cameras (2) Generation of Wide-Range Virtual Environment by Using Photorealistic Images ’ (Proceedings of the Virtual Reality Society of Japan, Second Annual Conference, pp. 67 70, 1997), etc.
A technique for reconstructing a geometric model of real space from real image data and expressing the model by conventional computer graphics can be mentioned as an example of a technique for expressing real space, which is obtained by capturing an image based upon real image data, as virtual space. However, there are limitations on the accuracy and realism of the model. Meanwhile, Image-Based Rendering (IBR) that expresses virtual space using real images without reconstructing a geometric model has become the focus of attention in recent years. The IBR technique is based upon real images and therefore makes it possible to express realistic virtual space. Further, a great amount of time and labor are required in order to create a geometric model of a broad space such as that of a city or town. An advantage of the IBR technique, however, is that such an expenditure of time and labor is unnecessary.
In order to construct walk-through virtual space using the IBR technique, it is necessary to generate and present an image that conforms to the position of the observer in the virtual space and the line-of-sight direction of the observer. In such a system, therefore, each image frame of the real image data and a position in the virtual space are stored in correspondence with each other, a corresponding image frame is acquired based upon the position of the observer in virtual space and the line-of-sight direction of the observer, and the acquired frame is reproduced.
In a walk-through within such virtual space, an image frame corresponding to each viewpoint position is stored as a panoramic image, which covers a range broader than the viewing angle at the time of reproduction, in such a manner that the observer can look along a desired direction at each viewpoint position. At the time of reproduction, the panoramic image is read out based upon the viewpoint position of the observer in virtual space and part of the image is cut from the panoramic image and displayed based upon the line-of-sight direction. In a case where the path of a viewpoint position in virtual space is the same as the path of a vehicle on which a photographic device has been mounted, the observer can experience the sensation of driving the vehicle himself.
By so arranging it that annotations such as the names of buildings are combined and displayed with the buildings in a displayed image in a walk-through within such virtual space, the observer can be provided with more valuable information. In addition, signs or the like that are difficult to see can be displayed in easily recognizable fashion as annotations on a real image.
However, when it is attempted to display an annotation on an object such as a building at all times in a walk-through constructed by the above-described IBR technique, the object for which display of the annotation is desired may be concealed by another object, and in such case which object the annotation belongs to cannot be identified. Alternatively, the correspondence between an annotation and an object may be misconstrued. Further, in a case where the distance between the viewpoint position in virtual space and the object is great, the object will appear small and hence there is the possibility that the object will be concealed by the annotation.